The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Exhaust systems for automobiles are used to filter and reduce the emission of nitrogen oxides (NOx). Several technologies have been developed to decrease NOx emissions. The technologies include passive methods that use catalysts and active methods, such as electrochemical catalysis and photocatalytical, plasma, and laser techniques. For example, rich-burning engines use a three-way catalyst to reduce NOx emissions. The three-way catalyst typically includes a combination of noble metals deposited on a stabilized alumina carrier. The three-way catalyst works with a feedback system that directs an engine to blend air and fuel in stoichiometric proportions. By controlling an air-to-fuel ratio of the engine, hydrocarbons (HCs) and carbon monoxide (CO) are produced to convert NOx and HCs to carbon dioxide, water, hydrogen gas, and/or nitrogen gas.
The NOx reducing performance of the three-way catalyst, which is the standard NOx abatement technology for gasoline engines, deteriorates rapidly in the presence of oxygen. Thus, this technology is ineffective in controlling NOx emissions in lean exhaust gases. Approaches that control NOx emissions in lean-burn engines, i.e., engines such as diesel engines that burn fuel in excess oxygen, include catalytic decomposition of nitrogen monoxide (NO) units and selective catalytic reduction (SCR) units.
The decomposition of NOx in a diesel exhaust requires high activation energy and thus is inhibited. Therefore, a catalyst is necessary to lower this activation energy in order to facilitate the decomposition. Various catalysts have been used to decompose NOx, including precious metals, metallic oxides, zeolites-based catalysts and copper ion-exchanged zeolite.
Although the use of a catalyst aids in the reduction of the activation energy, present diesel engines at startup and when moderately operated tend to exhibit cool exhaust temperatures. This delays effectiveness of an SCR unit. An SCR unit needs to reach a minimum operating temperature to effectively filter NOx. Typically, an SCR unit does not efficiently clean an exhaust until several minutes after an engine is started.